Welcome! Every trick at a skate park – rolling, speeding up, pushing off – is controlled by just three rules. Isaac Newton wrote them down in 1687. Today you’ll learn all three. Then you’ll be a skate park coach and work out which rider speeds up fastest.
🔑 Key words for today
- Force – a push or a pull. Measured in newtons (N).
- Resultant force – the overall force when all the forces are added up.
- Mass – how much matter something has. Measured in kilograms (kg).
- Acceleration – how quickly something speeds up. Measured in m/s².
- Inertia – the way objects keep doing what they’re already doing.
- Interact – when two objects push or pull on each other.
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: Newton’s three laws of motion – Skate Park Physics.
- ⭐ Core = everyone does this (about 40 minutes). 🚀 Stretch = extra challenge if you finish.
- Write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Copy the blue 📝 boxes into your book – they are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. state Newton’s three laws of motion
2. use F = ma to calculate acceleration
3. (🚀) use the second and third laws together.
⭐ Step 1: Starter (5 minutes)
You’re standing on a bus. The driver brakes suddenly and you lurch forwards – but nobody pushed you. Why?
Show answer
The brakes slow the bus down, but nothing has slowed you down yet. So your body keeps moving forwards. This is Newton’s first law – and it’s why we wear seatbelts!
📝 Copy into your book
Moving objects KEEP MOVING unless a force stops them.
⭐ Step 2: Read and write (8 minutes)
Copy these three questions. Then read the text and answer them.
- What happens to a moving object if the resultant force on it is zero?
- What equation links force, mass and acceleration?
- When you push on something, what does it do to you?
📖 Read: Newton’s three laws
Law 1 – things keep doing what they’re doing. If the resultant force on an object is zero, a still object stays still. A moving object keeps moving at the same speed in the same direction. A skateboard slows down on its own only because friction and air resistance are forces acting on it.
Law 2 – F = ma. A resultant force makes an object accelerate. The equation is force = mass × acceleration (F = ma). A bigger force gives a bigger acceleration. A bigger mass gives a smaller acceleration.
Law 3 – every push has a push back. When two objects interact, they push on each other with equal and opposite forces. When a skater pushes the ground backwards with their foot, the ground pushes the skater forwards.
Show answers
(a) It keeps moving at the same speed in the same direction.
(b) F = ma (force = mass × acceleration).
(c) It pushes back on you with an equal force in the opposite direction.
📝 Copy into your book
LAW 1: Resultant force = 0 → object stays still OR keeps moving at a constant velocity.
LAW 2: force = mass × acceleration F = m a
LAW 3: When two objects interact, they exert EQUAL and OPPOSITE forces on each other.
⭐ Step 3: Using F = ma (8 minutes)
👀 Watch me first
A skater and board have a mass of 50 kg. The resultant force on them is 100 N. What is their acceleration?
1. Equation: F = m × a, so a = F ÷ m
2. Numbers in: a = 100 ÷ 50
3. Answer with unit: a = 2.0 m/s²
⭐ Now you try: a 40 kg skater and board have a resultant force of 120 N. What is their acceleration?
Show answer
a = 120 ÷ 40 = 3.0 m/s². Less mass and more force, so a bigger acceleration than the example.
⭐ Step 4: Main task – Skate Park Coach (12 minutes)
You coach at a skate park. Four riders push off from the start line. The table shows each rider’s mass (with their board, bike or skates) and the resultant force on them.
| Rider | Riding | Mass | Resultant force |
|---|---|---|---|
| Aisha | Skateboard | 40 kg | 80 N |
| Ben | BMX | 60 kg | 150 N |
| Cara | Scooter | 30 kg | 90 N |
| Dev | Roller skates | 50 kg | 75 N |
⭐ Core: use a = F ÷ m to calculate each rider’s acceleration. Who speeds up fastest?
Show answer
Aisha: 80 ÷ 40 = 2.0 m/s²
Ben: 150 ÷ 60 = 2.5 m/s²
Cara: 90 ÷ 30 = 3.0 m/s²
Dev: 75 ÷ 50 = 1.5 m/s²
Cara speeds up fastest. Ben has the biggest force, but he also has the biggest mass.
⭐ Core: the skate park wants a poster explaining how skaters move. Write three sentences – one for each of Newton’s laws.
✏️ Sentence starters
Law 1: A skater who stops pushing slows down because ______. With no friction they would ______.
Law 2: Cara accelerates fastest because ______.
Law 3: To move forwards, a skater pushes the ground ______, and the ground pushes ______.
Show a model poster
Law 1: A skater who stops pushing slows down because friction and air resistance act against them. With no friction they would keep rolling at the same speed forever.
Law 2: Cara accelerates fastest because she has the biggest force for her mass (a = F ÷ m = 3.0 m/s²).
Law 3: To move forwards, a skater pushes the ground backwards, and the ground pushes the skater forwards with an equal force.
🚀 Stretch: Aisha (40 kg) and Dev (50 kg) stand still on their boards and push each other apart. Aisha pushes Dev with a force of 100 N. What force does Dev put on Aisha? Calculate both accelerations. Who rolls away faster?
Show answer
Dev pushes Aisha with 100 N in the opposite direction (third law).
Aisha: a = 100 ÷ 40 = 2.5 m/s²
Dev: a = 100 ÷ 50 = 2.0 m/s²
Aisha rolls away faster – the forces are equal, but she has less mass.
Step 5: Exam practice (10 minutes)
⭐ Q1 (3 marks) A resultant force of 12 N acts on a 3.0 kg ball. Calculate its acceleration.
Show model answer
a = F ÷ m (1)
a = 12 ÷ 3.0 (1)
a = 4.0 m/s² (1)
⭐ Q2 (3 marks) A cyclist stops pedalling on a flat road. She slowly comes to a stop. Explain why, using Newton’s laws.
✏️ Start with: “Friction and air resistance…” then “so the resultant force…” then “so she…”
Show model answer
Friction and air resistance still act on her (1), so there is a resultant force acting backwards, against her motion (1), so she decelerates until she stops (1).
🚀 Q3 (2 marks – Higher) A resultant force of 18 N gives a trolley an acceleration of 0.60 m/s². Calculate the trolley’s inertial mass.
Show model answer
Inertial mass = F ÷ a = 18 ÷ 0.60 (1) = 30 kg (1)
✅ Core score out of 6 (Q1–Q2). Stretch: out of 8.
⭐ Step 6: Exit ticket (5 minutes)
- Newton’s first law means…
- A heavier skater accelerates more slowly because…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes. Want more? Try the Newton’s laws questions or the Crash Test Engineer lesson.
